1. Field of the Invention
The present invention relates to a load-lock unit and, more particularly, to a load-lock unit for transferring a wafer between atmospheres having different pressures.
2. Description of the Related Art
When a semiconductor wafer is to be transferred between a process chamber for processing a semiconductor wafer in a vacuum atmosphere and the outside (atmospheric pressure), a load-lock unit is used to improve the operation efficiency by shortening the time required for evacuation. With this load-lock unit, for example, to load the wafer in the process chamber from the outside, the wafer is first placed in a load-lock chamber, the interior of the load-lock chamber is evacuated to a predetermined pressure, the load-lock chamber is opened to communicate with the atmosphere in the process chamber, and then the wafer is loaded into the process chamber.
A semiconductor wafer has a crystal orientation. Thus, when a wafer is to be processed or tested, not only its central position but also its orientation or direction of arrangement sometimes needs to be aligned.
For example, in an ion implantation unit, as shown in FIG. 1, ions generated by an ion generator I disposed in a terminal unit T are deflected by an analyzing magnet M and sequentially implanted in a wafer W on a turntable 1a (upright during ion implantation) in a process chamber through an acceleration tube A. Each wafer must be placed on the turntable 1a from the outside to be correctly aligned.
For this purpose, conventionally, as shown in FIG. 2, wafers W are transferred one by one to an aligning unit OD from a carrier 3 disposed at a predetermined position outside the process chamber 1 by a transfer robot R1 on the outer air side. An orientation error and a positional error of the center of each wafer are detected by the aligning unit OD, and two correcting steps for correction of the orientation and central position are performed to correct the errors, thus positioning the wafer W. Then, the wafer W in the aligning unit OD is transferred to a load-lock unit 2 by the transfer robot R1. The load-lock unit 2 is evacuated, and the wafer W is transferred to the turntable 1a from the load-lock unit 2 by a transfer robot R2 on the process chamber 1 side.
In such a conventional method, however, when the wafers are transferred into the load-lock unit one by one from the carrier, they must pass through the aligning unit, resulting in an increase in number of wafer handling times. Thus, damage to the wafer tends to occur or particles of dust tend to attach the wafer, leading to a decrease in yield. At the same time, as the number of handling times is increased, the loading time is prolonged, decreasing the processing throughput of the load-lock unit.
Regarding the aligning unit, a servo mechanism for moving the wafer in X and Y directions is need to correct, e.g., the central position of the wafer. As a result, the aligning unit becomes complicated and costly and requires an additional installation space for the servo mechanism.